% prop4.tut
% Classical implication definition II (A=>B) => ~A|B

classical proof classImpDef : (A=>B) => ~A|B =
begin
  [ A => B;
    [ ~(~A | B);
      [ ~A;
	~A | B;
        F ];
      A;
      B;
      ~A | B;
      F ];
    ~A|B ];
  (A=>B) => ~A|B
end;